Control component structure

ABSTRACT

A technique is provided for engaging and disengaging burnable poison rods from the spider in a fuel assembly. The cap on the end of each of the burnable poison rods is provided with a shank that is received in the respective bore formed in the spider. In one illustrative embodiment, the shank is deformed to firmly secure the rod and attached shank in the spider bore. Pressing the shank in the direction of the bore axis overcomes the deformation to release the particular rod from the spider.

This application is a continuation of application Ser. No. 952,522,filed Oct. 18, 1978, now abandoned.

TECHNICAL FIELD

This invention is directed to improvements in nuclear reactor fuelassemblies, and, more particularly, to methods and apparatus forsecuring and removing burnable poison rods from the spider in a fuelassembly, and the like.

BACKGROUND ART

To produce useful power from nuclear processes it is necessary toassemble a sufficient concentration of fissionable uranium, or othersuitable material, in a physical configuration that will sustain acontinuous sequence of energy-producing reactions. This assembly, orreactor core, transfers the heat that is generated in the fissionreactions to a working fluid. Frequently, pressurized water flowingthrough the core at high velocities is used for this purpose.

Because the heat, vibration and radiation that is generated within apower reactor core creates a generally hostile environment, thestructural integrity of the core components is an importantconsideration. Consequently, pressurized water power reactor coresfrequently are comprised of groups of fuel assemblies that are arrangedin a generally right circular cylindrical configuration. Each fuelassembly, moreover usually comprises an array of about two hundred longslender fuel rods that are parallel to and spaced from each other. Eachof these fuel rods contains a stack of generally cylindrical uraniumdioxide pellets in which the uranium provides the fissionable fuel forthe power reactor.

These fuel assemblies are not limited to fuel rods but also support anumber of other components. For example, instrumentation tubes forobserving temperature and neutron flux conditions within the core; endfittings and fuel element grids for stabilizing the fuel assemblycomponents; and control rods and control rod guide tubes for regulatingthe power output from the reactor through the selective absorption offission inducing neutrons within the reactor core often are made a partof the fuel assembly structure.

Clearly, the neutron distribution will vary from place-to-place withinthe reactor core. Illustratively, near the core perimeter it can beexpected that the neutron population will be small relative to thecenter of the reactor core because the concentration of neutronproducing uranium is lower at the core perimeter than it is in thecenter of the core. Neutrons at the core perimeter can "escape" from thecore more readily through the core surface than they can from the centerof the core, further tending to reduce the neutron concentration nearthe reactor core surface. Because heat generation within any specificportion of the reactor core is related to the neutron population withinthat portion, there is a definite tendency to produce highertemperatures at the center of the reactor core than at its margin. Thisinclination toward producing local temperature maxima in differentregions within the reactor core is generally undesirable for a number ofreasons. Primarily, the reactor is designed for core operation that willnot exceed a predetermined temperature. If this core temperature isreached in just one or in a few local points within the reactor core,the over-all heat generating potential of the core can not be realized.This effect results because the temperatures elsewhere in the core mustbe kept to lower values in order not to exceed the design temperature atthose limited points or "hot spots" in which the maximum designtemperature has been reached.

Accordingly, in the simplified example under consideration, over-allreactor power can be increased if the neutron population (and hence,heat) in the central portion of the core is depressed and the neutronpopulation in the larger volume that characterizes the peripheralreactor core annulus is allowed to increase. In this way, by"flattening" the power distribution in the reactor core, the coreactually is able to generate more power than it would be able togenerate if the neutron concentration, temperature and power was allowedto reach a peak in the center of the core, or in some other location, asthe case may be. In order to achieve this "flat" power distribution, ithas been the practice to insert "burnable poison" rods in the fuelassemblies. Typically, a burnable poison rod is a tube filled with amaterial that has a very high probability for absorbing neutrons. Forexample, a sintered dispersion of boron carbide in an alumina matrix issuitable for this purpose.

Neutrons, absorbed in this manner by means of the material within therod are, in effect, withdrawn from the fission and power generationprocess. And so, to "flatten" the power distribution with a reactorcore, burnable poison rods are concentrated in those fuel assembliesthat are located in the central portion of the reactor core.

Depending on a number of subtle effects it also might be advisable toprovide burnable poison rod concentrations in other portions of thereactor core in which specific design or operational features producelarge local neutron populations.

Not only must the fuel assembly support all of these diverse structuralcomponents in spite of the generally hostile environment within areactor core, but the fuel assembly also must be capable of the somewhatconflicting need for swift and easy disassembly. For instance, it shouldbe noted that fuel assemblies become radioactive after exposure in anoperating reactor core. This radiation is so intense that inspection andrepair can be accomplished only with remote handling equipment behindadequate radiation shielding.

Consequently, because disassembly procedures are expensive and timeconsuming, the need for a sturdy, yet readily dismountable structure isof considerable commercial importance.

The burnable poison rods that are used in many fuel assemblies are apart of this structural picture. Generally, the burnable poison rodsthat have characterized the prior art are mounted for movement in adirection that is parallel to the longitudinal orientation of the fuelrods. A "spider," in the form of a centrally disposed hub from which anumber of arms radiate often is used to couple the burnable poison rodstogether for longitudinal movement relative to the balance of the fuelassembly. This motion is required to permit the power reactor operatorto insert or withdraw the burnable poison rods from the reactor core inresponse to power flattening needs.

Eventually, lumped burnable poison rods must be removed from operation,packaged in a cask that provides adequate radiation shielding and thenshipped for disposal at a suitable site. It is desirable to remove eachof the rods from the spider in order to economize on the volume of theshipping cask. Unfortunately, removing the burnable poison rods from thespider is complicated and potentially hazardous for a number of reasons.The irradiated poison rods have developed, after sufficient irradiationin the reactor core, an internal gas pressure. The cladding or tubing inwhich the burnable poison has been loaded also becomes quite brittle asa result of a period of irradiation. The burnable poison rods usuallyare joined to their respective spider arms by means of threadedfasteners. In these circumstances, the most frequently suggestedtechniques for removing the rods from the spider are by means ofshearing or sawing. Sawing the rods permits these rods to be handledmore gently--an important consideration in view of the gas pressurewithin the rods--but the sawing process generates radioactive chips.Shearing overcomes this problem to a large extent, but does neverthelessresult in undesirably rough handling.

There is, then a need for some suitable means for joining burnablepoison rods to the spider arms in a manner that is proof against thereactor core environment but permits these rods to be removed from thespider simply, swiftly, and delicately.

SUMMARY OF THE INVENTION

These and other problems that have beset the prior art are overcomethrough the practice of the invention. More specifically, a pin issecured to one of the transverse ends of a burnable poison rod. The pinis seated in a bore that is formed in the spider arm appropriate to therod under consideration. Suitable means are provided for securing therod to the spider arm. Illustratively, upsetting the end of the pin tosecure the pin to the adjacent portion of the spider arm is suitable forthis purpose.

In accordance with a salient feature of the invention, the burnablepoison rod is separated from the spider arm by applying a force in adirection that is coincident with the longitudinal axis of the rod andits associated pin. The force is of sufficient magnitude to press thepin out of the spider arm. Typically, the longitudinal force presses theupset portion against the spider arm and, essentially extrudes the upsetportion of the pin through the bore in the spider arm that houses thepin.

In these circumstances, although considerable force may be applied topress the pin through the spider arm bore, it will be noted that theforce is applied in a longitudinal direction relative to the pin. Thisfeature of the invention not only protects the brittle, pressurizedburnable poison rod from risk of fracture or other damage due to therough handling that characterizes shearing but also avoids producing amass of radioactive chips if the rod is to be sawed free from thespider.

There are a number of more subtle advantages that accrue through thepractice of the invention. Manufacturing costs, for example, arerelatively low in comparison to the threaded fasteners that havecharacterized the prior art. Joint performance, moreover, is superior atthe temperatures commonly reached within nuclear reactor cores. Forinstance, the greater thermal expansion of the spider arm (frequentlymade from stainless steel) relative to the Zircaloy pin on the burnablepoison end plug causes undesirable axial stress that must be compensatedfor by other means. This is achieved in the invention by a correspondingradial loosening, thereby reducing the chance for generating undesirablethermal stresses in this region. Without this automatically providedstress relief, these thermally induced forces otherwise might cause thepin to shear and thus permit the burnable poison rod to become anundesirable loose object within the reactor core.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawing and descriptivematter in which there is illustrated and described a preferredembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a typical spider and burnable poison rod combination;

FIG. 2 illustrates in full section a typical embodiment of a portion ofa pin in accordance with principles of the invention;

FIG. 3 shows in broken section a pin installation in a portion of aspider that characterizes features of the invention; and

FIG. 4 is a drawing in broken section for a technique for removingburnable poison rods in a manner that further characterizes theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a more detailed understanding of the invention, attention is invitedto FIG. 1. As illustrated, a spider 10, formed from stainless steel, orthe like, is provided with a centrally disposed, generally cylindricalhub 11 that has a longitudinal axis 12. The spider 10 has a number ofspider arms that extend radially from the hub 11 in a plane that isperpendicular to the axis 12. For purposes of simplified description,only spider arms 13, 14, 15, 16, 17, 20 and 21 which are viewed in theplane of of FIG. 1 of the drawing are shown.

A group of burnable poison rods 22, 23, 24, 25, 26, 27 and 30 areattached to their respective spider arms. All of these burnable poisonrods are long, slender tubes in which the longitudinal axis of each ofthese tubes is parallel with the longitudinal axis 12 of the hub 11. Asillustrated in more detail with respect to the burnable poison rod 30, ahollow tube 31 forms the outer surface of the rod. On longitudinal endof the tube 31 is sealed by means of a solid plug 32 that is welded orotherwise secured in the open tube end. Within the tube 31 and bearingagainst the plug 32 is a stack of short cylindrical burnable poisonpellets 33. The individual cylindrical pellets in the stack 33 arepressed together in a longitudinal direction by means of a spring 34which is lodged between the last pellet in the stack 33 and a plug 35that is secured in the other open end of the hollow tube 31.

Turning now to FIG. 3, it can be seen that the plug 35 is provided witha chamfered end 36 that provides a lead in for a cylindrical portion 37.The inside diameter of the hollow tube 31 matches the outside diameterof the cylindrical portion to an extent that a snug fit is achieved.

As shown in the drawing, the cylindrical portion, 37 terminates in aradially protruding shoulder 40 that engages the transverse end of thehollow tube 31 to which it is welded. In the illustrative embodiment ofthe invention, the diameter of the hollow tube 31 in order to establisha flush relationship between an exposed cylindrical portion 41 of theplug 35 that extends beyond the transverse end of the tube 31 and theouter surface of the tube.

A transverse flange 42 that is spaced longitudinally from the shoulder40 terminates the exposed cylindrical portion 41 of the plug 35. A stem43 protrudes in a longitudinal direction from the central portion of thetransverse flange 42. Most of the shank that forms the stem 43 is formedfrom solid metal stock. A longitudinal bore 44 is formed, however, inthe stem 43 for a depth that extends from the free end of the stemthrough a distance that is about half of the stem length.

A portion of a stem 45 as manufactured, and before full installation ona spider arm 46, is shown in FIG. 2. Thus the stem 45 has a generallycylindrical exterior configuration. Bore 47, that extends from the freeend of the stem 45 to about half its length, also is of a generallycylindrical shape with the exception of the bottom of the bore which isformed into the shape of a shallow cone.

It will be recalled that the stem 45 is received in the spider arm 46.To accomplish this end, the spider arm 46 is provided with a generallycylindrical passageway 50 that has a longitudinal axis 51 which isessentially coincident with the longitudinal axis of the stem 45 and theburnable poison rod (not shown in FIG. 2). In accordance with acharacteristic feature of the invention, an enlarged recess 52 is formedwithin the passageway 50. Note in this respect, that the recess 52overlaps only a portion of the bore 47 in the stem 45.

A truncated conical surface 53 affords a transition from the recess 52to a bore 54 which forms the balance of the passageway 50. As shown inFIG. 2, the bore 54 has a slightly larger diameter than the outsidediameter of the stem 45 that is lodged in the passageway 50.

In operation, the stem 45 is inserted into the appropriate passageway50. The stem is mechanically deformed through hydroforming, mechanicalflaring or other suitable means, to match the contour of the truncatedconical surface 53 and the enlarged recess 52 and to provide a designed"weak point" for poison rod removal, as illustrated in FIG. 3. In thismanner, the burnable poison rod is secured to the associated spider armwithout making use of costly and difficult to handle weld fittings, andthe like.

It will be recalled, moreover, that different metals often are used forthe spider arm and for the burnable poison rod. The truncated conicalsurface 55, however, provides a tapered surface that alleviates theeffect of the different coefficients of thermal expansion. Typically, asthe temperature increases, the exemplary stainless steel spider armexpands in an axial direction more than the poison rod end plug 35(usually Zircalloy). In this manner, the axial stresses that otherwisewould have been generated are alleviated because of the radial looseningof the parts. Thus, because the spider arm also expands radially morethan the plug 35, the conical interface between the adjacent deformedportion of the stem 43 and the conical surface 53 shifts axiallydownward. The two parts remain in intimate contact radially and axially,but the strain in the stem 45 has been materially reduced relative tothe stresses that otherwise would have been generated. Thus, although atight joint is maintained throughout the entire range of reactoroperating temperatures, the structure nevertheless provides apredetermined design "weak point" to facilitate removal of the burnablepoison rod as described subsequently in more complete detail.

A further salient feature of the invention is shown in FIG. 4. Asillustrated, a plug 56 seals an end of a burnable poison rod 57. Theplug 56 has a stem 60, the longitudinal axis of which is coincident withlongitudinal axis 61 of the burnable poison rod 57.

A bore 62 extends longitudinally through about half the length of thestem 60. As shown in the drawing, this bore has been expanded or upsetthrough about half of its length to produce an enlarged cavity thatenables the outer diameter of the adjacent portion of the stem 60 tobear against an enlarged recess 64 in a spider arm 65.

The bore 62 has a truncated conical taper 63 that also provides atransition to a smaller diameter bore portion 66. The smaller diameterbore portion 66, moreover, terminates in a cone 67.

In accordance with a salient feature of the invention, a mandrel 70 ispressed into the bore 62 in the direction of arrow 71. The mandrel 70,preferably formed in the shape of a slender, cylindrical metal shaft,has a diameter that is slightly smaller than the diameter of the smallerdiameter bore portion 66. The longitudinal axis of the mandrel 70 isgenerally coincident with the longitudinal axis 61 of the burnablepoison rod 57.

To remove the burnable poison rod 57 from the spider arm 65 in a mannerthat will avoid undesirable shearing or sawing procedures it is onlynecessary to press the mandrel 70 in the direction of the arrow 71 withforce sufficient to extrude the stem 60 through small diameter recess72, thereby freeing the stem 60 from the spider arm 65.

Naturally although not shown in the drawing, a group of mandrels can beganged together on a plate These mandrels, aligned with and insertedinto corresponding bores on respective burnable poison rods mounted in aspider, can press all of the rods out of the spider in one operation. Inthis way, a potentially hazardous, expensive and tedious process ofshearing or sawing each burnable poison rod from the spider is avoided.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A control componentstructure comprising:a spider having a plurality of arms, at least onespider bore formed in said plurality of arms, said spider bore includingan enlarged recess and a small recess with an upright truncated conicalsection forming a transition from the enlarged recess to the smallrecess, and a burnable poison rod including a tube terminating in atransverse end, a plug with a chamfered end that leads into acylindrical portion, said cylindrical portion of the plug snugglyfitting within the tube and terminating in a radially protrudingshoulder which engages the transverse end of the tube to which it iswelded, and a stem protruding in the longitudinal direction from thecentral portion of the shoulder having a longitudinal stem boreextending through about half of the length of the stem, at least part ofthe stem which defines the stem bore is fixed within the truncatedconical section, the enlarged recess and the small recess of the spiderby outward deformation of that portion of the stem in order toreleasably attach the rod to the spider, said stem adapted tosubstantially reshape itself by movement of the rod in a longitudinaldirection with respect to the spider bore, while maintaining thestructural integrity of the poison rod and maintaining the structuralintegrity of the spider.
 2. The control component structure according toclaim 1 wherein the spider is made of material which has a greatercoefficient of thermal expansion than that of the stem.
 3. The controlcomponent structure according to claim 1 wherein the material of thespider is stainless steel.
 4. The control component structure accordingto claim 1 wherein the material of the stem is zircalloy.